When two or more substances mix completely to form a single uniform phase, the resulting mixture is a solution. This process involves two components: the solute (the substance being dissolved) and the solvent (the substance present in the greater amount, such as water). Understanding how much solute a solvent can hold is central to chemistry, as all solutions have a limit to the amount of material they can hold.
Defining Saturated Solutions
A saturated solution represents the precise point where a solvent holds the maximum possible concentration of a solute at a specific temperature. If a person were to keep adding salt to a glass of water, eventually a portion of the salt would stop dissolving and instead settle on the bottom of the container. This undissolved solid indicates that the solution has reached its solubility limit, or its saturation point.
Despite appearing static, a saturated solution is actually in a state known as dynamic equilibrium. In this state, two opposing processes occur simultaneously and at equal rates, resulting in no net change to the solution’s concentration. The rate at which the solid solute dissolves is exactly balanced by the rate at which the dissolved solute crystallizes back onto the solid material.
The concentration of a saturated solution is defined by this constant back-and-forth exchange, which establishes the maximum amount of solute that can remain dissolved. The concentration achieved at this equilibrium measures the substance’s solubility under those conditions. Any material added beyond this limit will simply remain in its solid form, as the solvent can no longer accommodate the molecules into its structure.
The Three States of Solution Concentration
Saturated solutions are understood by contrasting them with the two other states of concentration: unsaturated and supersaturated. These three categories define the relationship between the actual amount of dissolved solute and the maximum amount the solvent can hold at a given temperature. An unsaturated solution contains less than the maximum amount of dissolved solute.
If more solute is added to an unsaturated solution, it will dissolve immediately, showing the solvent still has capacity to incorporate additional material. This condition is stable because the system is far from its dissolving limit. For instance, a small amount of sugar fully dissolved in a cup of tea creates an unsaturated solution, allowing more sugar to be easily added.
The third state, a supersaturated solution, is the least common and the most unstable of the three. A supersaturated solution contains more dissolved solute than the solvent can theoretically hold under normal conditions. This unusual state is typically achieved by first creating a saturated solution at a high temperature, then carefully cooling it down without allowing the excess solute to precipitate.
This state is highly sensitive to disturbance. Introducing a small piece of solid solute, called a seed crystal, or jarring the container, can cause the excess dissolved material to rapidly crystallize out until the system reverts back to its stable saturated state. The instability of supersaturated solutions distinguishes them from the equilibrium of a saturated solution.
Factors Influencing the Saturation Point
The saturation point of any solution is not an absolute value but depends upon several external factors. The most significant variable that changes a substance’s solubility, and therefore its saturation point, is temperature. For most solid solutes, like salts or sugars, increasing the temperature of the solvent increases the amount of solute that can be dissolved.
This is why more sugar can be dissolved in hot water than in cold water before the solution becomes saturated. Conversely, for gases dissolved in a liquid, such as oxygen in water or carbon dioxide in soda, solubility decreases as the temperature rises. This effect is why warm soda quickly goes flat, as the carbon dioxide gas escapes from the solution.
Pressure is another factor, primarily affecting the solubility of gaseous solutes. Increasing the pressure of a gas above a liquid forces more of that gas into the solution, thereby raising its saturation point. This principle is used in commercial carbonation, where carbon dioxide is forced into a beverage under high pressure to achieve a higher concentration than possible at standard atmospheric pressure.